It is important to have a focus monitoring method which allows the reliable determination of the focus status of a lithographic process. The publication “Optimization and Characterization of the Blazed Phase Grating Focus Monitoring Technique” by Bill Roberts and Gerhard Kunkel; Infineon; discloses a method to increase focus resolution. The Blazed Phase Grating has been proven to be a suitable vehicle for on wafer aberration monitoring, but needs ex-situ correlations to determine the coefficient values. The phase grating method is introduced in production as a routine focus monitor. The accuracy of this method allows us to provide detailed analysis about the levelling capability and stability of all exposure tools. The testing is based on an optical microscope that can process more than 6000 data point per hour with very high accuracy. Beside the daily focus monitoring, which only takes five minutes, it allows fast full wafer analysis for chuck evaluation, wafer edge behaviour and other signatures that detract yield.